1. Field of the Invention
This invention relates to a class of equipment known as protected equipment. There are many examples of protected equipment in systems, particularly electronic systems, and more particularly communications systems.
2. Description of the Prior Art
There are many different types of equipment in a class called protected equipment. This class of equipment attempts to achieve an increase in reliability and availability of its features to its customers by structuring its implementation. This structure is realized as redundant sets of modules which are subparts of the equipment and which implement functions of the equipment. This equipment is structured and controlled such that there are redundant modules for each feature or group of features of importance for the equipment. In this structure there are at least two modules which can implement each important function. These modules are normally referred to as redundant modules or redundant module sets. Protected equipment solutions are constructed of one or more sets of these redundant modules. The premise for this structure is that if the currently active one of these sets fails, the other identical set which was not active can be quickly substituted for the failed set and the equipment can resume providing service to its customer.
A methodology and mechanism is required to control and communicate information regarding the control of the function that decides which of the redundant sets is going to be the active set. When this function is structured to operate without intervention of separate equipment or personnel entity, it is commonly referred to in the art as automatic protection switching (APS). When used in reference to communication facility protection, it has as a subset what is commonly called equipment protection switching (EPS) which is used for the protection of the equipment modules. The switching decisions can be made by a central controller, however, a mechanism to communicate with a central controller and the queuing associated with the central controller multi-tasking solutions can add delay in the reconfiguration of the equipment to restore customer service which is lost when the active module of a redundant module set fails. Also, if the system controller fails or is removed, the ability to perform the reconfiguration process will be lost.
Automatic protection systems for network elements in the past have comprised, for example, redundant facility equipment wherein a detected failure in one of the redundant sides will cause a switch over from one redundant side to the other. In such a switch over the entire line of protection equipment on one side is substituted for the other, even if a failure occurred in only one module in the working channel. This approach is somewhat inflexible in that it fails to maximize the available equipment.
Moreover, the switching decision is typically made by the system controller and all the information relevant to the switch over decision then has to be sent to the system controller. Since the system controller services all the information in the system, a large number of connections are required to be made through the back plane and the time required to effect the switch over can be relatively long. In addition, all the elements on the data path need to be switched during a switch over.
Thus, a better automatic protection switching system is desired.
In a system that has a redundant pair of modules where one module is the active module and the other module is the standby module, each of the two modules comprises equipment such as an inexpensive programmable logic device or processor to provide a protection switching algorithm for determining, at the individual module level, whether a module is in the active or standby state and switching a module from a standby state to its active state when the module in the active state becomes defective. This reduces the number of wires that are required as module to module indicators to two. The processor can make the determination of which module is in the active state and which is in the standby state when the two modules are powered up simultaneously, when one module is inserted subsequent to the other, or where a module has been active and then fails or is unexpectedly removed.